We had earlier suggested a mechanism for the origin of frameshift mutations. We now propose tests for our model, and will attempt to relate the frequency of mutation to the base sequence at the site of the mutation. Our recent results led us to propose a new model for acridine mutagenesis and we propose tests of that model. Mutations in the bacteriophage T4 lysozyme will be examined; the frequency of mutation will be determined by selective methods that have been developed, while the base sequence will be examined by the analysis of amino acid replacements. In our model for the origin of frameshift mutations, additions or deletions of bases occur through the mis-alignment of short stretches of bases at sites in the DNA which contain repeating bases or base doublets, and which are bounded on one side by a single stranded gap. The frequencies of the deletion and addition of a base pair have already been measured at a particular site at which the number of identical sequential base pairs is varied from four to six. These measurements are now being repeated at another site. In another set of experiments the base sequences of mutated sites with unusually high reversion frequencies will be determined. In order to test our new model for acridine mutagenesis we plan to examine the mutagenicity of compounds which stack around bases but which are unable to intercalate into DNA.